The present invention relates to a process for the conversion of internal olefins having a carbon number in the range from 4 to about 30 to terminal or alpha-olefins of corresponding carbon number.
Olefins in the C.sub.4 to C.sub.30 range have well recognized commercial utility, for instance, in the synthesis of surfactants, lubricants, and plasticizers. From the standpoint of most recognized uses for the olefins, there is a definite economic incentive for the isomerization of olefins having an internal double bond position to alpha olefins with a terminal double bond.
Thermodynamically, however, isomerization in the reverse direction is favored, and many processes are known for the conversion of alpha olefins to internal olefins.
For the conversion of internal olefins to alpha olefins, U.S. Pat. No. 3,131,225 to A. J. Rutkowski and U.S. Pat. No. 3,173,967 to H. C. Brown describe the use of alkylborane compounds. Amoung the drawbacks to use of this process are relatively high temperatures and long reaction times and difficulties in recovering and recycling active alkylborane.
In the isomerization process of the present invention, use is made of certain cyclopentadienyl compounds of tantalum. J. Schwarz et al have reported (Angew. Chem. Int. Ed. Engl., 15 (1976), p.333) that cyclopentadienyl compounds of zirconium, of the form Cp.sub.2 Zr(H)Cl, can be applied to olefin isomerization. An alkylzirconium species is formed by reaction of an internal olefin with the Cp.sub.2 Zr(H)Cl. Subsequent cleavage of this species results in a net isomerization of the internal olefin starting material to alpha olefin. This route suffers substantial disadvantage, however, in the measures which must be taken to liberate the olefin. Schwarz et al found that olefin was not released from the alkylzirconium complex upon heating or upon treatment with another olefin, e.g., ethylene. Isomerized olefin was obtained only through treatment of the alkylzirconium species with trityltetrafluoroborate, a procedure accompanied by substantial conversion of the trityltetrafluoroborate to triphenylmethane by-product.
In U.S. Pat. No. 4,125,567, R. L. Kidwell et al also report that zirconium complexes of the same sort investigated by Schwarz et al can be applied to olefin isomerization. In apparent contradiction to the findings of Schwarz, Kidwell et al disclose that alpha olefin can be liberated from the alkylzirconium species by a treatment for exchange of the bound olefin with another olefin (for example, ethylene) of different carbon skeletal structure. It is suggested that formation of an alkylzirconium species from an internal olefin and subsequent exchange with another olefin results in a net isomerization of internal to alpha olefin. Experiments repeating the work of Kidwell et al have not indicated, however, that the overall process fails to achieve such an isomerization in any significant degree. Under the procedures of U.S. Pat. No. 4,125,567, the cyclopentadienyl-zirconium compound apparently accomplishes a separation of alpha olefins from mixtures with internal olefins, rather than a conversion of internal olefins to alpha olefins.
With specific regard to reagents useful in the process of the present invention, cyclopentadienyl-tantalum compounds are known materials (U.S. Pat. No. 3,288,829 to G. Wilkinson; M. L. H. Green et al, J. Chem. Soc. 4854 (1961); and A. H. Klazinga et al, J. Organometal. Chem., 157 (1978), 413). Such compounds are not known to be recognized for utility in olefin isomerization. The Klazinga publication does report that the reaction between Cp.sub.2 TaCl.sub.2 and either n-BuMgCl or s-BuMgCl yield the complex Cp.sub.2 Ta(H) (1-butene).